In our pipeline, LiDAR point clouds are firstly projected into a set of 2D range images [one can see Fig. REF  (c)], which is defined as [1]}
\(\left(\begin{array}{c}u \\v\end{array}\right)=\left(\begin{array}{c}\frac{1}{2}\left[1-\arctan (y, x) \pi ^{-1}\right] \quad w \\{\left[1-\left(\arcsin \left(z r^{-1}\right)+\mathrm {\theta }_{\mathrm {up}}\right) \mathrm {\theta }^{-1}\right] h}\end{array}\right)\) 
